(1) Field of the Invention
The present invention relates to a method and a device for piloting an aircraft.
The invention thus lies in the technical field of aircraft having a piloting assistance system. In particular, the invention lies in the technical field of a piloting assistance system that makes use of sensors delivering information about the movement of the aircraft, such as an angular velocity, an attitude, an acceleration, a ground speed, . . . , this information being delivered with given accuracy.
(2) Description of Related Art
The movement of an aircraft is usually controlled by moving control members of the aircraft. Each control member serves to contribute to controlling the three-dimensional position of the aircraft. In order to move the control members, the aircraft may possess actuators.
By way of illustration, the blades of a main rotor and the blades of a tail rotor of a helicopter constitute such control members. Servo-control type actuators then enable the pitch of the blades to be controlled. Furthermore, fuel metering type actuators serve to act on control members in the form of engines that drive the blades in rotation.
A piloting assistance system of an aircraft may comprise various piloting modes that make use of respective flight-control laws for controlling the actuators of the aircraft. A flight-control law serves to issue an order to at least one actuator as a function of the current values of various parameters and as a function of at least one target to be maintained. Amongst these parameters, a flight-control law may make use of at least one component of the ground speed of the aircraft in a terrestrial reference frame.
Numerous flight-control laws are known. For example, a flight-control law using a target seeks to maintain a particular target. Document FR 2 814 433 refers to such laws.
By way of illustration, a ground speed flight-control law for a rotary wing aircraft seeks to maintain a target ground speed that is given by means of a control moved by a pilot. When the pilot positions the control means in a central position, the pilot may for example be requesting a ground speed that is zero. The ground speed flight-control law then enables the aircraft to remain stationary, performing hovering flight.
In order to maintain a target as a function of ground speed, a piloting assistance system may make use of various speed measuring systems in order to evaluate at least one component of the current ground speed in a terrestrial reference frame.
A first speed measuring system is in the form of an inertial guidance platform. An inertial guidance platform is an instrument used on an aircraft in order to estimate its attitude, its speed, or indeed its position relative to a starting point.
An inertial platform is usually provided with an inertial measurement unit (IMU). The inertial measurement unit has numerous inertial sensors. An inertial measurement unit may in particular comprise three gyros used for measuring the components of an angular velocity vector about three axes. Furthermore, an inertial measurement unit may have three accelerometers for measuring the components of a specific force vector (a magnitude also known as g-force and equivalent to the load factor) along three axes of the aircraft relative to the terrestrial reference frame.
In addition, the inertial platform includes a computer connected to the inertial measurement unit. Where appropriate, the computer integrates in real time the measurements taken by the inertial measurement unit in order to determine the components of the ground speed vector of the aircraft, or indeed a pitching angle, a roll angle, and a heading angle of the aircraft, together with its position. More precisely, by integrating measurements from the gyros, the computer determines the attitude of the aircraft and thus its orientation at a given moment. Furthermore, by integrating accelerometer measurements, which may be given relative to a terrestrial reference frame external to the aircraft when the orientation of the aircraft is known, the computer determines the ground speed components of the aircraft, e.g. in the terrestrial reference frame. Finally, by integrating its speed, the computer determines the geographical position of the aircraft.
Specifically, the inertial sensors of the inertial measurement unit present measurement biases, which may indeed vary during a flight. Furthermore, such inertial sensors are subject to measurement noise. The electrical signals issued by the inertial sensors are also processed by electronic circuits, which may themselves introduce noise.
The measurement biases and noise then falsify the measurements taken. Inertial platforms are advantageous since such inertial platforms have very good availability, however, the errors to which they are thus subjected lead over time to drift in the measurements taken, and in particular in the integrated ground speed.
In order to optimize the operation of an inertial platform, the inertial platform may be provided with high-performance sensors, such as gyros that possess errors not exceeding a few hundredths of a degree per hour and accelerometers presenting errors not exceeding a few tens of millionths of terrestrial gravity. Such a high quality inertial platform is nevertheless very expensive.
A second system is in the form of a satellite positioning system.
Such a satellite navigation system comprises a receiver on board the vehicle that receives signals from a plurality of satellites belonging to a satellite constellation, the constellation being controlled by fixed infrastructure on the ground referred to as the “ground segment”. The combination constituted by the receiver, the constellation, and the ground segment constitutes a satellite navigation system, which can be referred to as a global navigation satellite system (GNSS). Several global navigation satellite systems are in operation at present, such as the system known as the global positioning system (GPS), and the Russian GLONASS system. The Chinese BEIDOU system, the Japanese QZSS system, and the European GALILEO system are at present under development or deployment.
A satellite navigation system makes it possible in particular to determine the position of an aircraft and the ground speed components of that aircraft in the terrestrial reference frame.
A general limitation on using satellite navigation systems in aircraft piloting systems lies in the possibility of multiple failures affecting a plurality of satellites simultaneously, or even a compete constellation.
Document FR 3 030 058, filed on Dec. 11, 2014 with the French patent office under the reference 14/02824 proposes specifically making use of a plurality of different satellite constellations.
Furthermore, a satellite navigation system is sensitive to external disturbances, such as atmospheric disturbances, for example.
Consequently, a first system consists in coupling an inertial platform with a satellite navigation system in order to obtain estimated components of a ground speed tending to limit measurement inaccuracies. The coupling may be achieved by using a Kalman filter. A Kalman filter makes it possible to obtain estimated components for a ground speed on the basis of ground speed components obtained with the satellite navigation system and on the basis of ground speed components obtained with the inertial platform.
The ground speed components obtained with the satellite navigation system are said to be “measured” insofar as the satellite navigation system does not present measurement errors that fluctuate over time.
Conversely, because of these errors, the speed components obtained from the inertial unit are said to be “predicted”.
Since the availability and the accuracy of data from satellite navigation systems and from inertial platforms are not perfect, controlling the aircraft with a flight-control law that makes use of such data can be difficult to achieve.
Furthermore, ever-higher levels of assistance in aircraft can lead to a certain loss of attention on the part of the pilot when an autopilot mode is engaged.
As a result, the operation of the autopilot system is monitored by a monitoring system.
The system for monitoring an autopilot system flying an aircraft performs the function of making safe the data sent to the autopilot module that actually generates the flight-control laws that are applied to controlling the actuators. It also has the function of issuing the information that needs to be brought to the attention of the crew, e.g. concerning a potentially degraded situation affecting the sensors in use. The consequence of sensor degradation on the pilot's workload is made transparent to the pilot for most of the time because of the redundancy among the sensors used and because of the management performed by the monitoring system, e.g. via false detection isolation and recovery (FDIR) algorithms. In contrast, after multiple failures, i.e. failures affecting a plurality of redundant sensors, the consequence on the pilot's workload can become quite significant.
In particular, it may happen that the value of a parameter used by a flight-control law becomes unavailable.
When a parameter used by the autopilot is declared to be unavailable by the monitoring system, the autopilot system is automatically disengaged so as to allow manual piloting to take over at the end of a time-out period. The transition between autopilot mode and manual piloting mode is accompanied by a major and relatively sudden change in the pilot's workload.
For example, during hovering flight undertaken by engaging an autopilot mode and making use of a flight-control law that maintains a target ground speed, a pilot may be concentrating attention on other aspects, such as a winching operation, for example. A malfunction of the system that measures the ground speed of the aircraft can lead to an unexpected movement of the aircraft that needs to be countered by the pilot, and, depending on the failure, might even lead to a sudden disengagement of the autopilot mode. The pilot then needs to take over control of the aircraft in part or in full.
The effect of degradations affecting sensors used by an automatic flight control system can therefore lead to the pilot changing piloting strategy, potentially shifting in a few seconds from an autopilot mode with a high level of assistance to a manual piloting mode.
Documents US 2014/027565, US 2014/316615, FR 2 978 858, and US 2013/261853 are also known.